Amorphous Ti(IV)-modified Bi₂WO₆ with enhanced photocatalytic performance

Abstract

Crystalline TiO₂ is a well-known oxide which can be used to improve the photocatalytic performance of other photocatalytic materials by a semiconductor-coupling strategy. However, compared with crystalline TiO₂, amorphous TiO₂-modified semiconductors have seldom been reported. In this study, amorphous TiO₂ (referred to as Ti(IV)) as a hole cocatalyst was used to modify the photocatalytic performance of a Bi₂WO₆ photocatalyst by a facile wet-chemical method, where metallic Pt as the electron cocatalyst was also coated on the Bi₂WO₆ surface to promote the interfacial electron transfer. It was found that the dual-cocatalyst modified Ti(IV)–Pt/Bi₂WO₆ photocatalyst exhibited an obviously higher photocatalytic performance than the blank Bi₂WO₆ and single-cocatalyst modified Pt/Bi₂WO₆ and Ti(IV)/Bi₂WO₆. Based on the present experimental results, we propose a synergistic effect of amorphous Ti(IV) and Pt to illustrate the enhanced photocatalytic activity of the Ti(IV)–Pt/Bi₂WO₆ photocatalyst, namely amorphous Ti(IV) works as a hole cocatalyst to rapidly transfer the photogenerated holes in the valence band of Bi₂WO₆, while Pt acts as an electron cocatalyst to rapidly transfer the photogenerated electrons on its conduction band. As a consequence, the transfer rate and the interfacial catalytic reaction of photogenerated electrons and holes were simultaneously accelerated, which resulted in improved photocatalytic performance of the Ti(IV)–Pt/Bi₂WO₆ photocatalyst. The above synergistic effect mechanism in Ti(IV)-modified Bi₂WO₆ photocatalysts can further be demonstrated by using a low-cost Fe(III) or Cu(II) electron cocatalyst. The present study suggests that amorphous Ti(IV) can act as a new and effective hole cocatalyst for the enhanced photocatalytic performance of photocatalysts, which provides an approach for the design and development of high-performance visible-light photocatalysts with amorphous oxides.

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1. Introduction

Semiconductor photocatalysis is a very effective and green technique in the treatment of wastewater and air pollution.^1–3 The study of visible-light-driven photocatalysts has been undertaken by an increasing number of researchers. In recent years, bismuth-based compounds with visible-light photocatalytic activity,⁴ such as Bi₂O₃,^5,6 BiVO₄,⁷ BiOI,^8,9 Bi₂MoO₆,¹⁰ and Bi₂WO₆,^11,12 have been successfully developed for organic pollutant degradation. Among them, Bi₂WO₆, one of the most important members in the Aurivillius family, has been demonstrated to be a promising candidate for visible-light-driven photocatalysts due to its high stability, nontoxicity, and narrower band gaps (2.69 eV).^13,14 However, the photocatalytic activity of Bi₂WO₆ is limited by its high recombination probability of photogenerated electron–hole pairs.¹⁵ As a consequence of the rapid recombination of photogenerated electrons and holes, most of the excited charge carriers are quenched before they can reach the surface to participate photocatalysis reaction. Therefore, numerous modification strategies, such as ion doping,^16–18 semiconductor coupling,¹⁹ noble metal deposition,^20,21 morphology controlling^22–25 and cocatalyst modifications^26–28 have been explored to improve the charge separation efficiency and enhance the photocatalytic performance of Bi₂WO₆ photocatalyst. On the basis of the present research, cocatalyst modification on the surface of Bi₂WO₆ has been considered as one of the most effective approaches to facilitate the separation of photogenerated electrons and holes.^29–33

Generally, the cocatalysts, which are used to enhance the efficiency of the charge separation, can be categorized into electron cocatalyst and hole cocatalyst.^34,35 After loading electron cocatalysts such as metal nanoparticles (Pt, Ag, Au etc.) on semiconductor, where a cocatalyst-semiconductor Schottky or Ohmic barrier will form, the cocatalyst nanoparticles will serve as an efficient sink for excited electrons.^36–41 On the other hand, hole cocatalysts such as transition metal oxides (RuO₂, IrO₂, CoO_x, NiO_x etc.) are employed to trap photogenerated holes and mediate the hole transfer process.^42,43 It has been reported that a Pt or Ag cocatalyst can enhance the separation efficiency of the photogenerated carriers of Bi₂WO₆, and then increase the photocatalytic activity.^20,44,45 However, to our best knowledge, there are seldom reports about hole cocatalyst depositing on Bi₂WO₆. Additionally, as photogenerated holes with strong oxidizing power have relatively lower transfer rate than that of photogenerated electrons, it is necessary to load effective hole cocatalysts on the surface of Bi₂WO₆ to promote the charge separation efficiency. At present, much attention has been paid on amorphous TiO₂ (a-TiO₂) nanoparticles owing to their special properties in heterogeneous catalysis.^46,47 For examples, Hu et al. presented that the deposition of a-TiO₂ can be used as a protective layer for photoanodes in solar cell water splitting while providing effective conducting holes.⁴⁶ Recently, we also found that a-TiO₂ can work as the hole cocatalyst to significantly improve the photocatalytic performance of CdS and Ag-based photocatalysts.^47,48 In comparison to its crystalline counterparts (rutile, brookite and anatase), a-TiO₂ has a great advantage for large-scale production due to the cost-effectiveness of growth techniques as well as its ability to form intimate contact interfaces with photocatalyst. Therefore, it is highly desirable to develop highly efficient photocatalysts by combining the crystalline Bi₂WO₆ nanoparticles and TiO₂ in amorphous phase to improve the photocatalytic activity.

Crystalline TiO₂ is a well-known oxide with suitable band edges which can be used to improve the photocatalytic performance of other photocatalytic materials by a semiconductor-coupling strategy.⁴⁹ In this case, the resultant TiO₂/Bi₂WO₆ heterojunction photocatalysts usually showed an improved photocatalytic performance.^50,51 Considering the completely different properties of amorphous TiO₂ with the crystalline phase, it is expected that amorphous Ti(IV) loaded on the photocatalyst surface can exhibit a different photocatalytic activity and mechanism. However, compared with the crystalline TiO₂-coupled system, the amorphous TiO₂-modifed semiconductors have been seldom reported. In this study, amorphous Ti(IV) and Pt co-modified Bi₂WO₆ photocatalyst is first presented by a facile two-step wet chemical method. As single-component Ti(IV) or Pt modification for Bi₂WO₆ photocatalyst can be carried out in a similar condition, it is feasible to co-modify amorphous Ti(IV) cocatalyst and conventional Pt electron cocatalyst on the surface of Bi₂WO₆ photocatalyst via Ti(SO₄)₂ impregnation and the following Pt photodeposition. In this case, amorphous Ti(IV) and Pt are the effective cocatalysts for the rapid transfer of photogenerated holes and electrons, respectively. Considering the simultaneous promotion for the transfer rates of photogenerated electrons and holes to reach specific reaction sites of photocatalyst, the photocatalytic activity of Ti(IV)–Pt/Bi₂WO₆ can be expected to be further improved. The photocatalytic activity of Ti(IV)–Pt/Bi₂WO₆ photocatalyst is evaluated by the decomposition of methyl orange and phenol solutions. Furthermore, the synergistic effect of amorphous Ti(IV) and Pt cocatalysts for the enhanced photocatalytic performance of Bi₂WO₆ photocatalyst is studied in detail.

2. Experimental details

All reagents are of analytical grade supplied by Shanghai Chemical Reagent Ltd. (P. R. China) and used as received without further purification.

2.1. Preparation of Bi₂WO₆ sample

The Bi₂WO₆ photocatalysts were prepared by a simple hydrothermal technique. In a typical synthesis procedure, 0.553 g of Bi(NO)₃ was dissolved in 20 mL of nitric acid solution (1 mol L⁻¹) and 0.188 g of Na₂WO₄ was then added into the above solution to form the suspension. Then, the resulting suspension was vigorously stirred for 1 h at room temperature. After stirring, the obtained sample was transferred into a Teflon-lined autoclave and kept at 180 °C for 12 h. Subsequently, the resulting precipitate was washed, filtrated with distilled water, and then dried at 60 °C for 12 h. Finally, the obtained sample was calcined at 450 °C for 3 h to obtain the Bi₂WO₆ sample.

2.2. Preparation of Ti(IV)/Bi₂WO₆ samples

The amorphous TiO₂-modified Bi₂WO₆ (Ti(IV)/Bi₂WO₆) photocatalyst was prepared by an impregnation technique. Briefly, 0.05 g of Bi₂WO₆ was dispersed into 10 mL of deionized water. Then, the proper amount of Ti(SO₄)₂ was added. After stirring at 75 °C for 1 h, the resulting samples were filtrated, rinsed with purified water, and finally dried at 60 °C to obtain the Ti(IV)/Bi₂WO₆ photocatalysts. The weight-ratio of amorphous Ti(IV) to Bi₂WO₆ in the Ti(IV)/Bi₂WO₆ photocatalysts were controlled to be 0, 1, 3, 5 and 10 wt%.

2.3. Preparation of Ti(IV)–Pt/Bi₂WO₆ samples

The Ti(IV)–Pt dual-cocatalyst modified Bi₂WO₆ (Ti(IV)–Pt/Bi₂WO₆) photocatalysts, in which weight-ratio of Pt to Bi₂WO₆ was fixed to be 1 wt%, were prepared by a photodeposition method in a H₂PtCl₆–MO solution system. Briefly, 50 mg of the above Ti(IV)/Bi₂WO₆ (5 wt%) composite was dispersed into the H₂PtCl₆ solution (133 μL, 10 mg mL⁻¹), and MO (10 mL, 20 mg L⁻¹) was added as an electron donor. Then, the solution was stirred continually in the dark for 1 h. Irradiation was carried out using a 300 W Xe lamp for 30 min. After irradiation, Pt loaded photocatalyst was washed with distilled water and dried at 60 °C overnight. Since the weight-ratio of amorphous Ti(IV) to Bi₂WO₆ in the Ti(IV)–Pt/Bi₂WO₆ photocatalysts were controlled to be 0, 1, 3, 5 and 10 wt%, the corresponding samples can be denoted as Bi₂WO₆ and Ti(IV)–Pt/Bi₂WO₆ (X wt%) (X = 1, 3, 5, and 10 wt%, respectively).

For comparison, the Pt-loaded Bi₂WO₆ (Pt/Bi₂WO₆) photocatalyst, whose weight-ratio of Pt to Bi₂WO₆ was 1 wt%, was also prepared by the same photodeposition method.

2.4. Modification of Ti(IV)/Bi₂WO₆ by Fe(III) or Cu(II) cocatalyst

The Ti(IV)–Fe(III)/Bi₂WO₆ photocatalyst was prepared by an impregnation technique. In a typical preparation, 0.1 g of Ti(IV)/Bi₂WO₆ powder was dispersed into 10 mL Fe(NO₃)₃ aqueous solution (0.05 mol L⁻¹) under constant stirring. After stirring at 60 °C for 2 h, the resulting sample was collected by filtration, rinsed with distilled water, and dried at 60 °C. Furthermore, the Cu(II)-loaded Ti(IV)/Bi₂WO₆ (Ti(IV)–Cu(II)/Bi₂WO₆) photocatalyst was also prepared by the same impregnation method, except that Cu(NO₃)₂ solution was utilized as the aqueous solution.

2.5. Characterization

The scanning electron microscopy (SEM) measurements were performed with a JSM-7500 field-emission scanning electron microscope (FESEM, JEOL, Japan) equipped with an X-max 50 energy-dispersive X-ray spectroscope (EDS, Oxford Instruments, Britain). Transmission electron microscopy (TEM) images were obtained on a JEM-2100F electron microscope (TEM, JEOL, Japan), using a 200 kV accelerating voltage. X-ray diffraction data were collected on a Rigaku Ultima III X-ray Diffractometer (Japan). UV-visible-light analysis was collected on a UV-visible spectrophotometer (UV-2450 SHIMADZU, Japan). Raman spectra were collected using an INVIA spectrophotometer (Renishaw, UK). X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ECALAB 250xi XPS spectrometer system (Mg Kα source). All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon.

2.6. Photocatalytic activity

The evaluation of photocatalytic activity of the prepared samples was performed by the photocatalytic decolorization of methyl orange (MO) aqueous solution or the photocatalytic decomposition of phenol solution. 0.05 g of the prepared sample was dispersed into 10 mL of MO solution (20 mg L⁻¹) or phenol solution (10 mg L⁻¹) in a culture dish with a diameter of ca. 5 cm. The solution was allowed to reach an adsorption–desorption equilibrium among the photocatalyst, MO and water before irradiation. Two 5 W 420 nm LED lamps (Shenzhen LAMPLIC Science Co.) were used as the visible light sources, and the average light intensity striking on the surface of the reaction solution is about 90 mW cm⁻². At a given interval, the concentration of MO or phenol was determined by a UV-visible spectrophotometer (UV-1240, SHIMADZU, Japan). As for the MO aqueous solution or phenol solution with a low concentration, its photocatalytic decomposition is a pseudo-first order reaction and its kinetics may be expressed as ln(c₀/c) = kt, where k is the apparent rate constant, and c₀ and c are the MO or phenol concentrations at initial state and after irradiation for t min, respectively.⁴⁷ For the repeated photocatalytic performance, the photocatalysts were first separated by centrifugation, washed with distilled water, and were then redispersed into the phenol solutions.

2.7. Photoelectrochemical measurements

The electrochemical impedance spectra (EIS) were carried out on a CHI 660E electrochemical workstation in a standard three-electrode configuration containing Na₂SO₄ (0.5 M) aqueous solution as the electrolyte, with a platinum wire as the counter electrode, Ag/AgCl as a reference electrode. The light source was provided by a 5 W LED (420 nm light source with a 90 mW cm⁻² power). The working electrodes were prepared on fluorine-doped tin oxide (FTO) conductor glass. Typically, 2 mg samples were ultrasonicated in 0.5 mL of anhydrous ethanol and 0.5 mL of Nafion D-520 dispersion (5%, w/w, in water and 1-propanol, Alfa Aesar) to obtain a suspension solution. The suspension was spun on the FTO glass with the side protected by Scotch tape and fully dried at 60 °C for 12 h. A copper wire was connected to the side of the working electrode. Finally, EIS was measured over the frequency range of 0.01 to 10⁵ Hz with an ac amplitude of 10 mV at the open circuit voltage.

3. Results and discussion

3.1. Controlled synthesis of amorphous Ti(IV)-modified Bi₂WO₆

The preparation of various photocatalysts such as Bi₂WO₆, Ti(IV)/Bi₂WO₆, Pt/Bi₂WO₆ and Ti(IV)–Pt/Bi₂WO₆ could be easily controlled by a facile wet-chemical route, as shown in Fig. 1. Firstly, Bi₂WO₆ precursor particles (Fig. 1a) were obtained via a hydrothermal reaction at a temperature 180 °C for 12 h and subsequently annealed at 450 °C for 3 h, while the Pt/Bi₂WO₆ (Fig. 1c) photocatalyst was prepared by a photodeposition method in a H₂PtCl₆–MO solution system. The Ti(IV)/Bi₂WO₆ sample could be prepared by an impregnation technique via the controlled hydrolysis of Ti(SO₄)₂ at a low temperature (75 °C). Since Ti(SO₄)₂ was hydrolyzed at relatively low temperature, amorphous TiO₂ phase was produced and coated on the surface of Bi₂WO₆ to produce Ti(IV)/Bi₂WO₆ samples. Finally, the preparation of Ti(IV)–Pt/Bi₂WO₆ samples was also prepared by a photodeposition method by using Ti(IV)/Bi₂WO₆ as the precursor.

Fig. 1 Schematic diagram illustrating the preparation of various samples: (a) Bi₂WO₆; (b) Ti(IV)/Bi₂WO₆; (c) Pt/Bi₂WO₆; (d) Ti(IV)–Pt/Bi₂WO₆.

3.2. Morphology and microstructures of amorphous Ti(IV)-modified Bi₂WO₆

The controlled preparation of above various Bi₂WO₆ samples can firstly be demonstrated by the FESEM, TEM and XRD results. Fig. 2 shows the FESEM images of the Bi₂WO₆, Ti(IV)/Bi₂WO₆, Pt/Bi₂WO₆ and Ti(IV)–Pt/Bi₂WO₆. It can be seen that the size of the as-prepared Bi₂WO₆ nanoparticles is in the range of 0.1–1 μm and their surfaces are smooth with lamellar structure (Fig. 2A). After surface modification by amorphous Ti(IV), the particle size of the resulting Ti(IV)/Bi₂WO₆ nanoparticles (Fig. 2B) has no obvious change. However, its surface morphology was covered with a plenty of small particles. To investigate the component of those nanoparticles in Ti(IV)/Bi₂WO₆ (5 wt%), the EDS analysis is performed and corresponding results are shown in the insert of Fig. 2B. It is found that in addition to the main Bi, W and O elements, the Ti element can also be observed and the weight ratio of Ti is about 3.78 wt%. As for the Pt/Bi₂WO₆ (Fig. 2C) samples, they also show very similar particle morphologies with the pure Bi₂WO₆, and the corresponding EDS spectrum (inset of Fig. 2C) indicates that the coexistence of Pt element with Bi₂WO₆ and the weight ratio of Pt is 0.63 wt%. For the dual-cocatalyst Ti(IV)–Pt/Bi₂WO₆ (Fig. 2D) samples, the FESEM results indicate that the simple modification with Pt or Ti(IV) only changes the surface morphology of Bi₂WO₆, and the EDS result shows that Ti element (3.34 wt%) and Pt element (0.60 wt%) can be found for Ti(IV)–Pt/Bi₂WO₆ (5 wt%) sample (insert of Fig. 2D). The phase structure of amorphous Ti(IV) and Pt on the surface of Bi₂WO₆ nanoparticles can be further demonstrated by high-magnification TEM analysis. High-magnification TEM image (Fig. 2E and F) of Ti(IV)–Pt/Bi₂WO₆ sample indicates that the Ti(IV) nanoparticles can be clearly found on the surface of Bi₂WO₆ nanoparticles.⁵² The results also reveal that Pt dispersed on the surface of Bi₂WO₆ nanoparticles and the diameter of the Pt is about 10 nm (Fig. 2E). No lattice infringes of TiO₂ are found, suggesting that TiO₂ is in amorphous state.

Fig. 2 FESEM images and EDS spectra (insert) of the various samples: (A) Bi₂WO₆; (B) Ti(IV)/Bi₂WO₆ (5 wt%); (C) Pt/Bi₂WO₆; and (D) Ti(IV)–Pt/Bi₂WO₆ (5 wt%). (E and F) The representative HRTEM images of sample (D).

The crystal phases of different samples are further identified by XRD patterns (Fig. 3). It is clear that all the diffraction peaks of the Bi₂WO₆, Ti(IV)/Bi₂WO₆, Pt/Bi₂WO₆ and Ti(IV)–Pt/Bi₂WO₆ samples can be indexed to be orthorhombic phase of Bi₂WO₆ (JCPDS Card: 39-0256). Furthermore, for those samples containing Ti(IV) and Pt, there is no obvious shift of Bi₂WO₆ characteristic peaks, suggesting that Ti(IV) or platinum is not incorporated into the lattice of Bi₂WO₆, but is deposited on the surfaces, which can be confirmed by the TEM images. Obviously, it is difficult to identify the related diffraction peaks of TiO₂ and Pt due to the low contents and high dispersion of amorphous Ti(IV) and Pt in the Ti(IV)–Pt/Bi₂WO₆ samples. These results clearly demonstrated that the co-loading of amorphous Ti(IV) and Pt co-catalysts have no effects on the crystal structures of the Bi₂WO₆. Moreover, modified samples show a comparable diffraction peak intensity and full width at half-maximum compared with the pure Bi₂WO₆ sample, suggesting that the crystallization and crystallite size of Bi₂WO₆ are not affected by the different modification processes owing to their mild conditions, in good agreement with the results observed in FESEM images. Therefore, based on the FESEM, TEM and XRD results, it is very clear that the Ti(IV)–Pt/Bi₂WO₆ samples coated by amorphous Ti(IV) cocatalyst have been successfully prepared via the above-mentioned method.

Fig. 3 XRD patterns of various samples: (a) Bi₂WO₆; (b) Ti(IV)/Bi₂WO₆ (5 wt%); (c) Pt/Bi₂WO₆; (d) Ti(IV)–Pt/Bi₂WO₆ (5 wt%).

To investigate the surface composition and chemical state of Ti(IV)–Pt/Bi₂WO₆ composite, the XPS spectra are measured and displayed in Fig. 4. Fig. 4A illustrates the XPS survey spectra of different samples in a wide energy range. It is clear that all the samples show the main binding energy peaks of Bi, W and O elements, which can be mainly attributed to the Bi₂WO₆ phase. Compared with the pure Bi₂WO₆ and Pt–Bi₂WO₆ samples, new XPS peaks of Ti(IV) element are found in the Ti(IV)/Bi₂WO₆ and Ti(IV)–Pt/Bi₂WO₆ samples after loading Ti(IV) in addition to the Bi, W, O and C elements. To further reveal Bi, W and Ti elements and their chemical states, the high-resolution XPS spectra of above samples are shown in Fig. 4B–D. In the high-resolution XPS spectrum of the Bi 4f (Fig. 4B), the 4f_7/2 and 4f_5/2 peaks located at 158.9 eV and 164.2 eV, respectively, which matches well with those from Bi₂WO₆.⁵³ The W 4f peaks (Fig. 4C) appeared at 35.1 eV and 37.2 eV were ascribed to W 4f_7/2 and W 4f_5/2, respectively, which were assigned to the W⁶⁺ oxidation state.⁵⁴ Compared with the pure Bi₂WO₆ (Fig. 4D-a), the Ti(IV)/Bi₂WO₆ (5 wt%) (Fig. 4D-b) shows the obvious XPS peaks of Ti element. The binding energies of Ti 2p are located at 458.7 eV (Ti 2p_3/2) and 464.7 eV (Ti 2p_1/2), which corresponded to Ti^4+ 55 and overlapped with the Bi 4d_3/2 peak at the binding energy of 465.2 eV. It is found that the peak intensity and position show no shift, indicating that the loading of Ti(IV) cocatalyst cannot significantly influence the surface structure of Bi₂WO₆ photocatalyst. The Pt 4f_7/2 and Pt 4f_5/2 peaks for Ti(IV)–Pt/Bi₂WO₆ are identified at 72.4 eV and 76.0 eV as shown in Fig. S1,† respectively, suggesting the presence of Pt.⁵⁶ Therefore, the results of XPS verified the existence of amorphous Ti(IV) and metal Pt, in good accordance with the data of FESEM and HRTEM. According to the element component analysis based on the XPS results (Table 1), it is clear that the Ti(IV) cocatalyst is 6.0–6.5 at% on the surface of Ti(IV)-modified Bi₂WO₆. These results further confirm that the Ti(IV) cocatalyst has been successfully loaded on the surface of Bi₂WO₆ photocatalyst.

Fig. 4 (A) XPS survey spectra and the high-resolution XPS spectra of (B) Bi 4f, (C) W 4f, (D) Ti 2p: (a) Bi₂WO₆; (b) Ti(IV)/Bi₂WO₆ (5 wt%); (c) Pt/Bi₂WO₆; (d) Ti(IV)–Pt/Bi₂WO₆ (5 wt%).

Table 1 Composition (at%) of the various samples according to XPS analysis

	C	O	Bi	W	Ti	Pt
Bi2WO6	39.28	44.62	9.66	6.44	0
Ti(IV)/Bi2WO6 (5 wt%)	39.35	45.8	5.29	3.48	6.08
Pt/Bi2WO6	36.23	47.28	9.81	6.68	0
Ti(IV)–Pt/Bi2WO6	41.08	44.43	5.11	2.92	6.42	0.04

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The optical properties of prepared photocatalysts are studied by using a UV-visible diffuse reflectance spectrometer. From Fig. 5, the Bi₂WO₆ presents its band gap absorption (ca. 460 nm), which was responsible for its visible-light-driven photocatalysis. The loading of Pt onto the Bi₂WO₆ does not cause obvious red shift compared with the parent Bi₂WO₆ due to a low content of Pt. After surface modification by Ti(IV) cocatalyst, the resultant Ti(IV)/Bi₂WO₆ photocatalysts present a similar absorption curve owing to its very limited amount of Ti(IV) cocatalyst on the Bi₂WO₆ surface. When dual cocatalysts are modified on the surface of Bi₂WO₆, Ti(IV)–Pt/Bi₂WO₆ composite catalyst exhibited higher light harvesting in the range of 470–600 nm in addition to its band gap absorption. The result may be ascribed to the combinational light absorption of Bi₂WO₆, TiO₂, and Pt. Therefore, the high light harvesting efficiency of Ti(IV)–Pt/Bi₂WO₆ opens the opportunity to enhance the UV-visible-light-induced photocatalysis.

Fig. 5 UV-visible diffused reflectance spectra of various samples: (a) Bi₂WO₆; (b) Ti(IV)/Bi₂WO₆ (5 wt%); (c) Pt/Bi₂WO₆; (d) Ti(IV)–Pt/Bi₂WO₆ (5 wt%).

3.3. Photocatalytic performance and mechanism of Ti(IV)-modified Bi₂WO₆

The photocatalytic performance of the samples was evaluated by photocatalytic decolorization of methyl orange (MO) aqueous solution, as shown in Fig. 6. It was clear that pristine Bi₂WO₆ nanoparticles showed low photocatalytic performance (Fig. 6a), which was attributed to the high recombination rate of photoinduced electron–hole pairs. When the Bi₂WO₆ nanoparticles were modified by amorphous Ti(IV) phase (5 wt%), the Ti(IV)/Bi₂WO₆ composite photocatalysts showed inconspicuous enhanced photocatalytic activity (Fig. 6b). After the photodeposition of Pt, the MO degradation efficiencies of Pt/Bi₂WO₆ were elevated because the Schottky barriers between semiconductor and metal suppressed efficiently the photocarriers recombination. Furthermore, when the Pt/Bi₂WO₆ nanoparticles were modified by amorphous Ti(IV) phase, the Ti(IV)–Pt/Bi₂WO₆ composite photocatalysts showed obviously enhanced photocatalytic activity (Fig. 6d–g). By optimizing the experimental conditions for the amorphous Ti(IV) phase, it was found that when the amount of Ti(IV) element was 5 wt%, the obtained Ti(IV)–Pt/Bi₂WO₆ (5 wt%) nanoparticles showed the highest photocatalytic activity (k = 1.23 × 10⁻² min⁻¹), which was a value larger than those of Bi₂WO₆ and Pt/Bi₂WO₆ by factors of 49.0 and 37.6, respectively. The enhanced photocatalytic activity benefited from the synergistic effect of amorphous Ti(IV) and Pt, which exploited the hole cocatalyst and the Schottky barriers between metal and semiconductor to prolong the lifetime of photocarriers. As the reduction reactions of photogenerated electrons and the oxidation reactions of photogenerated holes occur on the surface active sites, an optimal amount of cocatalyst can contribute to a rapid separation of photogenerated charges, resulting in an enhanced photocatalytic performance. Furthermore, the photocatalytic performance of Ti(IV)/Bi₂WO₆ photocatalysts decreased when the content of Ti(IV) cocatalyst further increased due to affecting the electron transfer and adsorption of O₂ on the Bi₂WO₆ surface when loading with large amount of Ti(IV) cocatalyst. This phenomenon was in good agreement with the widely reported cocatalyst-modified photocatalysts such as Ag₂O/Bi₂WO₆.¹⁵ To further investigate the performance stability of the Ti(IV)–Pt/Bi₂WO₆ photocatalyst, the circulating runs of phenol degradation were conducted and shown in Fig. 7. After five cycles, the rate constant of the photocatalytic decomposition of phenol solution still kept about 6.79 × 10⁻³ min⁻¹, indicating that the Ti(IV)–Pt/Bi₂WO₆ (5 wt%) had high stability and was not photocorroded during the photocatalytic process. The enhanced photocatalytic activity and long-time reusability of the Ti(IV)–Pt/Bi₂WO₆ composite photocatalyst made it a good candidate in the practical environment purification.

Fig. 6 The rate constant (k) of MO decomposition by various photocatalysts: (a) Bi₂WO₆; (b) Ti(IV)/Bi₂WO₆ (5 wt%); (c) Pt/Bi₂WO₆; (d) Ti(IV)–Pt/Bi₂WO₆ (1 wt%); (e) Ti(IV)–Pt/Bi₂WO₆ (3 wt%); (f) Ti(IV)–Pt/Bi₂WO₆ (5 wt%); (g) Ti(IV)–Pt/Bi₂WO₆ (10 wt%).

Fig. 7 The repeated photocatalytic performance of Ti(IV)–Pt/Bi₂WO₆ (5 wt%) for the decomposition of phenol solution.

It is very interesting and meaningful to investigate the potential photocatalytic mechanism of Ti(IV)–Pt/Bi₂WO₆ photocatalyst. It is clear that the Bi₂WO₆ photocatalyst exhibits a negligible photocatalytic performance for the decomposition of organic pollution (Fig. 6a), although it can absorb visible light as shown in the UV-visible spectrum. According to our previous study,¹⁵ the conduction band (CB) and valence band (VB) of Bi₂WO₆ were calculated to be ca. +0.3 V and +3.0 V (vs. SHE), respectively. It is well-known that the CB potential of a semiconductor should be more negative than that of a one-electron oxygen-reduction reaction (O₂ + H⁺ + e⁻ = HO₂, −0.046 V vs. SHE)⁵⁷ with the aim of promoting the efficient transfer of CB electrons to oxygen. Obviously, the CB electrons of Bi₂WO₆ possess a poor reduction power owing to its more positive potential (+0.3 V, vs. SHE) than the one-electron oxygen reduction. Therefore, the CB electrons in the Bi₂WO₆ cannot reduce oxygen effectively even loading Ti(IV) hole cocatalyst (Fig. 8a and b), resulting in a negligible photocatalytic performance. To promote the effective oxygen reduction, metallic Pt was deposited on the surface of Bi₂WO₆ nanoparticles by a light-induced deposition method. When Pt was grafted onto the Bi₂WO₆ surface to form Pt/Bi₂WO₆ photocatalyst, the photogenerated electrons on the Bi₂WO₆ CB could easily transfer to the Pt cocatalyst owing to the formation of a Schottky barrier between Pt and Bi₂WO₆ (Fig. 8c). As a consequence, the resultant Pt/Bi₂WO₆ photocatalyst exhibited excellent photocatalytic activity for the decomposition of MO solution (k = 3.4 × 10⁻³ min⁻¹).

Fig. 8 Schematic diagrams illustrating the possible photocatalytic mechanism of various photocatalysts: (a) Bi₂WO₆; (b) Ti(IV)/Bi₂WO₆; (c) Pt/Bi₂WO₆; (d) Ti(IV)–Pt/Bi₂WO₆.

When both of the Pt and Ti(IV) cocatalysts are deposited on the surface of Bi₂WO₆, it is clearly found that the photocatalytic performance of resulting Ti(IV)–Pt/Bi₂WO₆ photocatalyst can be further improved, which can be well explained by the synergistic effect of Pt and Ti(IV) cocatalysts. The Pt nanoparticles result in effective transfer of photogenerated electrons for oxygen reduction reaction, while Ti(IV) cocatalyst leads to effective transfer of photogenerated holes for oxidation reaction of organic substances. In other words, the Ti(IV)–Pt/Bi₂WO₆ photocatalyst cooperates the advantages of Ti(IV)/Bi₂WO₆ and Pt/Bi₂WO₆ to simultaneously accelerate the separation of photogenerated holes and electrons, and thus exhibits the highest photocatalytic performance (Fig. 8d). Hence, the above experimental results strongly support that the synergistic effect of Pt and Ti(IV) cocatalysts simultaneously accelerates the transfer rates of photogenerated electrons and holes for Bi₂WO₆ photocatalyst, thus accomplishing more efficient degradation of organic substances. To further demonstrate the synergistic effect of Pt and Ti(IV) cocatalysts for the effective charge transfer, the electrochemical impedance spectrum (EIS) was performed. Fig. S2† presents the EIS Nyquist plots of Bi₂WO₆, Ti(IV)/Bi₂WO₆, Pt/Bi₂WO₆ and Ti(IV)–Pt/Bi₂WO₆ samples under UV light irradiation. The smaller arc radius implies the higher efficiency of charge transfer. The arc radius for the Ti(IV)/Bi₂WO₆ and Pt/Bi₂WO₆ samples are smaller than that of the Bi₂WO₆, while the Ti(IV)–Pt/Bi₂WO₆ photocatalyst shows the smallest diameter, strongly demonstrating that the loading of Ti(IV) and Pt onto the Bi₂WO₆ surface can enhance the transfer efficiency of photogenerated electron and hole pairs.

It has been widely reported that crystalline TiO₂ can be used to improve the photocatalytic performance of Bi₂WO₆ by a semiconductor-coupling strategy.⁴⁹ For the semiconductor-coupling strategy, the electrons in the valence bands (VB) of both Bi₂WO₆ and TiO₂ are excited to their CB, while holes remain on their VB. Owing to the different CB and VB potentials of Bi₂WO₆ and TiO₂, the electrons in the CB of TiO₂ can transfer to that of Bi₂WO₆, while holes in the VB of Bi₂WO₆ can move to that of TiO₂. As a consequence, the probability of electron–hole recombination can be greatly reduced, thus improving the photocatalytic performance. In this study, the amorphous Ti(IV)-modified Bi₂WO₆ shows a completely different mechanism with the above semiconductor-coupling route. Amorphous Ti(IV) works as an effective hole cocatalyst to rapidly transfer the photogenerated holes in the valence band of Bi₂WO₆, leading to the significantly improved performance in Ti(IV)–Pt/Bi₂WO_6.

3.4. The low-cost Ti(IV)/Bi₂WO₆ photocatalysts by loading Fe(III) or Cu(II) cocatalyst

Apparently, modification of a photocatalyst by electron and hole cocatalysts is a quite effective strategy to obtain high photocatalytic performance. However, most of the widely reported electron or hole cocatalysts are noble metals (Pt and Au) or their oxides (Ru₂O and IrO₂).^37,58 Considering practical applications, it is very necessary to develop low-cost and earth-abundant materials as the high-performance cocatalyst to improve photocatalytic efficiency of semiconductor photocatalysts. According to our previous results, it is found that the low-cost and earth-abundant Fe(III) and Cu(II) clusters can function as the highly efficient electron cocatalysts to greatly improve the photocatalytic performance of various photocatalytic materials.^59–61 Therefore, in the present study, the Fe(III) and Cu(II) clusters are also loaded on the Ti(IV)/Bi₂WO₆ photocatalyst surface to develop the low-cost and high-performance Ti(IV)–Fe(III)/Bi₂WO₆ and Ti(IV)–Cu(II)/Bi₂WO₆ photocatalysts, respectively. Photocatalytic experimental results indicated that the Ti(IV)–Fe(III)/Bi₂WO₆ and Ti(IV)–Cu(II)/Bi₂WO₆ displayed obviously higher photocatalytic activity than the Bi₂WO₆ and Ti(IV)/Bi₂WO₆ (Fig. 9A). The reason for enhanced photocatalytic performance is that Fe(III) or Cu(II) can quickly capture the photogenerated electrons from Bi₂WO₆ and Ti(IV) can effectively transfer the photogenerated holes from Bi₂WO₆ simultaneously, resulting in a rapid separation of photogenerated electrons and holes (Fig. 9B). As for the Ti(IV)–Fe(III)/Bi₂WO₆ photocatalyst, for example, the photogenerated electrons on the Bi₂WO₆ CB can easily transfer to the Fe(III) cocatalyst owing to its more positive potential of Fe³⁺/Fe²⁺ (0.771 V, vs. SHE)⁵⁷ than the CB of Bi₂WO₆ (+0.30 V, vs. SHE), which promoting the rapid separation of photo-generated electrons and holes, and resulting in an enhanced photocatalytic performance (Fig. 9B). In this case, the Fe(III) cocatalyst can accept a photogenerated electron to form Fe(II), which is unstable and easily becomes Fe(III) through the multi-electron reduction of oxygen under ambient conditions, namely, the Fe(III) can be well recovered via the effective oxidation of Fe(II) by oxygen. A similar multi-electron oxygen reduction was also found on the Cu(II) cocatalysts. Owing to the rapid capture of photogenerated electrons by Fe(III) or Cu(II) cocatalyst, the recombination rate of photogenerated charges in the bulk and surface of Bi₂WO₆ photocatalysts can be significantly decreased, leading to an improvement of photocatalytic performance of Ti(IV)–Fe(III)/Bi₂WO₆ and Ti(IV)–Cu(II)/Bi₂WO₆. In this study, even though the performance of photocatalysts loading with Cu(II) or Fe(III) cocatalysts is lower than those with Pt cocatalyst, considering the large amount of Cu and Fe elements in natural resources, the current Cu(II) and Fe(III) cocatalysts are possible to be widely applied in various photocatalytic materials to improve their photocatalytic performance.

Fig. 9 (A) Photocatalytic decomposition of MO solution by various photocatalysts: (a) Bi₂WO₆; (b) Ti(IV)/Bi₂WO₆ (5 wt%); (c) Ti(IV)–Fe(III)/Bi₂WO₆ (5 wt%); (d) Ti(IV)–Cu(II)/Bi₂WO₆ (5 wt%). (B) Schematic diagrams showing the possible photocatalytic mechanism of Ti(IV)–Cu(II)/Bi₂WO₆ or Ti(IV)–Fe(III)/Bi₂WO₆.

4. Conclusions

In summary, metallic Pt nanoparticles and amorphous Ti(IV) cocatalysts were loaded on the surface of Bi₂WO₆ to prepare the highly efficient Ti(IV)–Pt/Bi₂WO₆ photocatalyst by a simple photoreduction-impregnation method. It was found that the metallic Pt nanoparticles function as an electron cocatalyst and amorphous Ti(IV) as a hole cocatalyst. Compared with the pure Bi₂WO₆, Pt/Bi₂WO₆, and Ti(IV)/Bi₂WO₆, the photocatalytic performance of Bi₂WO₆ photocatalyst could be greatly improved by the surface loading of Pt nanoparticles and Ti(IV) cocatalyst. On the basis of the experimental results, a possible synergistic effect mechanism of Pt nanoparticles and Ti(IV) cocatalyst was proposed to account for the improved photocatalytic performance of Ti(IV)–Pt/Bi₂WO₆ photocatalyst, namely, the metallic Pt nanoparticles cause an obviously fast transfer rate of photogenerated electrons, while the Ti(IV) works as a hole cocatalyst and an effective active site for the following oxidation reaction of organic substances to reduce the recombination rate of photogenerated electrons and holes. The present work can provide some new insight for the smart design and preparation of new high-performance photocatalytic materials.

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (51472192, 21277107 and 21477094), and the program for new century excellent talents in university (NCET-13-0944). This work was also financially supported by program for Fundamental Research Funds for the Central Universities (WUT 2015IB002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10616a

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